Desensitization reduces amplitudes of quantal end-plate currents after a single preceding end-plate current in mouse muscle

Abstract

While in membrane patches nicotinic channels from end-plates desensitize with time constants of 10-100 ms and recover with time constants of 100-200 ms, doubts remain as to whether such rapid reactions are also found in intact neuromuscular junctions. Therefore, the desensitization effected by a single end-plate current (EPC) on monoquantal EPCs (qEPCs) was studied. Using a published reaction scheme for the adult end-plate receptor, the desensitizing effects of an EPC on a following one were simulated. Twenty milliseconds after an EPC, a subsequent qEPC was reduced to 90% of control, and to 70-80% if acetylcholine esterase (AChE) was blocked. EPCs were elicited and recorded through a macropatch electrode. Series of four EPCs (control qEPC, conditioning EPC, test qEPC 1 and 2) were repeated several thousand times and the amplitudes and decay time constants of the qEPCs were evaluated. A highly significant average depression of the qEPC to 96.4% of control was found 20 ms after the conditioning EPC; if the AChE was blocked the depression after 40 ms was to 76% of control. Under both conditions, the time constant of recovery from desensitization was below 100 ms. The time constant of decay of the qEPC, taudecay, tended to be slightly shortened during depression in the presence of AChE. If AChE was blocked, taudecay was lengthened to 130-140% of control after a preceding EPC, and this lengthening outlasted the depression of the EPC amplitude by a second. These changes in taudecay are not predicted by the channel reaction scheme, and possible additional modulatory effects are discussed.